Extend Task.rate_limit to accept float values

Celery is enforcing rate_limit integer values, but the kombu TokenBucket implementation
fill_rate value is casted to float anyway, therefore remove this limitation.
Fixes mongodb tests
Updates Changelog
Bumps version to 3.0.13
Now depends on Kombu 2.5.4
Add myself to Contributors (pull request 1142)
Fixes pickling of configuration modules when using execv/fork emulation.  Closes #1126